Unless rural Africans benefit far more from ecotourism the "shocking" decline of the continent's remaining lions will continue, a British scientist says.

Fewer than 20,000 lions may now survive in the whole of Africa, he says, though they do not face immediate extinction.

The greatest threats to the species his researchers found are sport hunting and conflict with farmers over livestock.

The lions appear to be declining very fast in many of the remoter parts of Africa, outside the tourist spotlight.

The scientist is Professor David Macdonald, director of WildCRU, Oxford University's Wildlife Conservation Research Unit.

If they were all in your sitting room, 20,000 lions might sound a lot, but we're talking about an entire continent

Professor David Macdonald, Oxford University

In a lecture at the Zoological Society of London he announced the results of his team's five-year study of lion conservation, which has concentrated on fieldwork in Zimbabwe and Botswana.

The estimate of 20,000 lions or fewer compares with a population put at about 200,000 in the early 1980s.

The researchers studied the impact of sport or trophy hunting in Hwange national park in Zimbabwe, surrounded by hunting concessions where the parks department allocates an annual quota: the hunters traditionally target male lions.

Professor Macdonald said their findings suggested the levels of hunting there were not sustainable. Of the adult males the team tagged or collared, 63% were shot by hunters in the surrounding area.

The resulting low density of male lions is exacerbated by the hunters' habit of shooting juvenile males when they find no mature adults.

Impossible pressure

This means males move widely, and may have ranges of 1,000 square kilometres, about three times the size of a lioness's range. So it is likelier they will leave the protection of the park and move into hunting areas.

Unusually, the Hwange lions also associate with more than one pride of females, meaning they are less able to protect the lionesses and their cubs.

Lionesses are left unprotected

WildCRU estimates there are about 42 adult male lions in Hwange, where between 1998 and 2002 the hunting quota in the concessions was set at 63 lions.

It says the number shot annually far exceeded the recommended sustainable level of 4-10% of the adult males. The parks department is considering lowering the quotas.

It compares the conflict in Botswana between lions and farmers with the hunting which led to the extermination of wolves, lynx and bears in the UK centuries ago.

WildCRU says: "African carnivores today are facing a fate alarmingly parallel to our long-departed carnivores, caught between the needs of a human population... and the predators' own considerable need for space and resources."

In the four years the team spent in Botswana's Makgadikgadi national park, it says, "lions were poisoned, trapped and shot, but never appeared to die from disease, starvation or injury."

WildCRU says some lions follow the seasonal migrations of zebra and wildebeest, but most seem to stay behind to ambush stray domestic livestock.

Not persuaded

The answer, it says, is to make more wild prey available for the lions (through discouraging hunting), and fewer domestic animals.

How the clashes often end

Consistent vigilance by the herders is critical, but hard to encourage, because they rarely perceive any benefit to themselves or their communities from the tourists attracted by the lions and other wild animals.

Professor Macdonald told BBC News Online: "Ecotourism may work for glitzy areas like the Okavango or Serengeti.

"It's a wonderful hope, a generator of revenue, but it's unrealistic to expect it to do everything.

"Local communities get very little benefit from it, but they must. The herders don't see the link between lions and tourists, because they don't see the money.

Frail flagship

"The lions' decline is shocking, because it suggests they're a great deal more frail than we might have thought.

"If they were all in your sitting room, 20,000 lions might sound a lot, but we're talking about an entire continent.

"And there can be no animal in the world more emblematic of wilderness and conservation than the lion.

"If even they have been decimated in a couple of decades, that may tell us something about what's happening to the less conspicuous and emblematic creatures."